EVOLUTION OF CEPHEIDS .1. THE EFFECTS OF AN ENHANCED C-12(ALPHA,GAMMA)O-16 RATE

Models of intermediate-mass stars (4, 5, 6, 7, 8, and 9 M⊙ with Y = 0.28, Z = 0.02) have been calculated with the 12C(α, γ) 16O rate of Caughlan et al. Although the rate for the 12C(α, γ) 16O reaction has been recently redetermined (Caughlan and Fowler) and found to be almost the same as it was in Fowler, Caughlan, and Zimmerman, the rate is uncertain within a factor of 2. In this paper we explore the influence of an enhanced 12C(α, γ) 16O rate on the evolutionary behavior of intermediate-mass stars in the H-R diagram by using the Caughlan et al. rate. The enhanced rate extends the tip of the blue loop in the HR diagram, allowing the 5 and 6 M⊙ models with a standard Population I composition to re-enter the Cepheid strip during core Heburning, unlike the models calculated using the Fowler, Caughlan, and Zimmerman rates.